An electric valve, for example included in a converter for conversion between alternating current and high-voltage direct current (HVDC converter), comprises a usually large number of mutually series-connected semiconductor devices in the form of thyristors. A control system, located at ground potential, for the converter generates a firing order for the valve and a control system for the valve, also located at ground potential, generates as a result of the received firing order a firing signal for each one of the thyristors included in the valve. These firing signals are received by a firing channel which is associated with each thyristor and which transmits the firing signal to an electronic unit associated with each thyristor. The electronic unit, which is at the potential of the thyristor, comprises, in the case with electrically fired semiconductor devices, circuits for converting a firing signal received as a light signal into an electric firing pulse which is applied to the gate of the thyristor, as well as an indicating unit. Where the thyristots are of a directly light-fired type, the electronic unit consists of an indicating unit only, which is then not connected to the gate of the respective thyristor. The indicating signal generated by the indicating unit is transmitted via an indicating channel to ground potential and is used to ensure, in a manner known per se, that the gate of a thyristor is not supplied with a firing pulse unless its off-state voltage in the forward direction has attained a predetermined value, adapted for a rapid and safe firing, as well as to indicate, by its occurrence, that the respective thyristor is not short-circuited. For the latter purpose, the indicating signal is supplied to a monitoring device for the valve or the converter, whereby the occurrence of an indicating signal also means a confirmation that the respective indicating channel is in operation. The absence of indicating signals for a semiconductor position is recorded in the monitoring device or in some storage medium connected thereto. Both the firing and the indicating channels are usually made as optical fibre links and are provided at their end points with members for conversion between electric and optical signals.
Further, the electronic unit comprises circuits, arranged in a manner known per se, for achieving a voltage-controlled firing of the thyristor in the event that firing in the intended manner by a firing pulse emanating from a firing signal generated by the control system fails to occur. The voltage-controlled firing is initiated when the off-state voltage of the thyristor in the forward direction exceeds a certain level.
For a general description of the technical background within the technical field mentioned, reference is made to .ANG.ke Ekstrom: High Power Electronics HVDC and SVC, EKC--Electric Power Research Center, The Royal Institute of Technology, Stockholm, 1990.
With the valve in the off-state, each thyristor takes up part of the voltage across the valve whereby the voltage division between the individual thyristors is determined by a voltage divider, comprising resistors and capacitors, connected in parallel with the thyristors. The valve is usually so dimensioned that in the event that one or a few individual thyristors, for example because of an internal short circuit, should have no voltage-absorbing ability, the remaining thyristors during operation under normal voltage conditions are still able to block voltages occurring across the valve. However, a monitoring of the operation of the thyristors included in the valve is still necessary such that faulty units can be replaced during planned maintenance work. According to the prior art, the monitoring is carried out in the manner described above by observing the indicating signals transmitted to the monitoring device, whereby the absence of an indicating signal, indicating a fault such as a short circuit in the respective thyristor or a fault in the indicating channel, is recorded together with an indication as to which semiconductor position has been found to lack indicating signal.
However, commonly known systems for the above-mentioned monitoring does not provide any information as to whether the thyristor has been fired in the intended manner by supplying to the gate of the thyristor a firing pulse emanating from a firing signal generated by the control system, or, in the absence of such a firing pulse, by voltage-controlled firing. Thus, a fault in the firing channel for a thyristor cannot be discovered in this way.